1. Field of the Invention
This invention relates to a composite material which has a substantially reduced incidence of defects after forming, and to a method of producing it. The composite material comprises at least one layer of plain-carbon steel, at least one layer of stainless steel, and between said layers, a layer of steel containing 0.5 percent or more of silicon.
2. Description of the Prior Art
Pressure-bonded composite members are used in a variety of applications. Cookware is often drawn from composite material comprised of plain-carbon and stainless steel. Plain-carbon steel is a better heat conductor than is stainless steel. On the other hand, stainless steel has a more attractive appearance and is stain-resistant to a variety of contaminants. Accordingly, a composite of the two metals is most desirable, as it combines the advantages of each metal.
Deep-drawn articles made from pressure-bonded composite materials of the kind indicated above have displayed surface defects which have become evident during and after drawing. These defects include an "orange peel" surface, wavy or wrinkled surfaces and surface ruptures which are exceedingly detrimental as they cannot be satisfactorily removed by such techniques as buffing or grinding.
It is known, for example, from U.S. Pat. No. 3,693,242, and U.S. Pat. No. 3,956,809, that in the production of such composite materials, an improvement in respect to the avoidance of defects may be obtained by using a plain-carbon steel to which there has been added an appropriate amount of a carbide-forming element such as titanium or columbium. In the above-mentioned patents it is hypothesized that the occurrence of defects is promoted by migration of carbon from the plain-carbon steel into the part of the stainless steel layer immediately adjacent thereto. The above-mentioned patents contain the observation that a migration of carbon can and often does occur in a direction opposite to the carbon gradient, i.e., from a material of lower carbon content to one of higher carbon content. The direction of carbon migration is dependent upon the attainment of a condition of lower free energy, and not strictly upon concentration gradients. Those skilled in the art will understand that with the approach of the two above-mentioned patents, using a plain-carbon steel which has a composition modified to contain an appropriate amount of a strong carbide-forming element, the carbon present in the plain-carbon steel tends to be present in the form of precipitates, rather than being in solution in the iron and correspondingly more readily available for migration. Those skilled in the art will also appreciate that there are advantages to be obtained if it is not necessary to resort to the use of a plain-carbon steel which has a particular modified composition containing the above-indicated appropriate amount of carbide-forming element; it is advantageous to be able to use, for the plain-steel layer, an ordinary plain-carbon steel, of a kind which is more generally available in greater quantities, and is, moreover, susceptible of being diverted to other uses if not needed for the production of composite articles of the kind indicated above.
In the industry of making specialty-steel products, it is known to manufacture various types or grades of non-oriented or grain-oriented silicon-containing electrical steel, and that these are not infrequently produced in the form of strip having a thickness on the order of 0.007 to 0.015 inch. Such steel regularly contains in excess of 0.5 percent of silicon, and commonly on the order of 3 percent silicon. Moreover, in the electrical-steel art, it is well known that carbon is generally detrimental to the desired electrical properties, and consequently, such steels are often made with an aim carbon content of approximately 0.03 percent or lower. Moreover, the process for making a high-performance grade of grain-oriented silicon steel is a complicated, exacting, multi-step procedure involving a succession of cold-rollings and annealings, in the course of which there is necessarily produced a certain quantity of flat-rolled silicon-bearing steel of suitably low carbon content, perfectly satisfactory for use in the practice of the present invention, but not capable of being sold as high-performance electrical steel. Commonly, any specialty-steel manufacturer capable of producing of AISI type 304 stainless steel, whether for the cladding in a composite product of the kind mentioned above or for other purposes, has been likely also to be engaged in the production of such electrical steels.
The usual manipulative steps are known in the art of the making of a pressure-bonded composite material as hot-rolled or later as cold-rolled. First, it is known, in general, to form a sandwich containing desired layers of the materials to be bonded, and to work in a non-oxidizing or reducing atmosphere, using vacuum, inert gas, or hydrogen, supplying heat by various means and applying pressure by various means, such as roll stands, platens or explosives. It can be taken as known that a composite material so formed may then be further cold-rolled to a desired gauge in one or more stages, using, as necessary, annealing treatments at a temperature in excess of 1800 degrees Fahrenheit.
The prior art further includes the article of L.S. Darken about diffusing of carbon in austenite in the Transactions of the AIME, 1949, Volume 180, Pages 430-438. This article presents data which confirm the existence of the phenomenon that carbon may diffuse from a first material of lower carbon content across an interface into a second material of higher carbon content. In addition, it includes data that would intend to make it unobvious to select a silicon-bearing steel for the composition of a layer adjacent to one into which it is desired to avoid having carbon be introduced. To be more specific, it gives data to show that a layer bearing 3.80 percent silicon and an overall carbon content of 0.49 percent, placed adjacent to one containing 0.05 percent silicon and an overall carbon content of 0.45 percent, came to have, after heating and diffusing, an interface with 0.315 percent carbon in the former and 0.566 percent carbon in the latter. This indicates that carbon will leave a steel in which it has higher activity to enter into one of the lower activity, even if the latter has a higher carbon content. This higher activity can be caused by richer silicon levels.
It is maintained that the prior art has not, prior to the present invention, made it obvious to persons of ordinary skill in the art what would happen with the use of a relatively thin silicon-steel interleaving layer between a layer of plain-carbon steel and one of an austenitic stainless steel like AISI Type 304, let alone that using such an interleaving layer affords a way to overcome orange peel and other defects, and without resort to modifying the composition of the plain-carbon steel by incorporating therein carbide-former elements like titanium and columbium.